1. Field of the Invention
This invention relates to a process for separating contaminants from the exhaust gases generated from combustion of fossil fuels, incineration, furnaces or gas streams of chemical processes, and more particularly, to the removal of nitrogen and sulfur oxides from an exhaust gas stream.
2. Description of the Prior Art
Nitrogen oxides (NO.sub.x) and sulfur oxides (SO.sub.x) are the main air pollutants found in the flue gases from combustion sources and chemical plants. The systems for removal of SO.sub.x gases by dry or wet scrubbing have reached an advanced stage of development. Other processes based on regenerative adsorbents are also available. Dry technologies are based on adsorption and wet technologies are based on absorption.
It is well known in the art to remove nitrogen oxides from flue gases by a number of dry and wet processes. Dry processes generally utilize catalytic decomposition or adsorption, while wet processes normally utilize absorption technology. A majority of the dry flue gas treatment (FGT) processes utilize catalytic decomposition or homogeneous decomposition at high temperatures. Dry flue gas treatment processes are normally carried out after the combustion process. Sometimes CaO/CaCO.sub.3 is added in the fluidized bed during the combustion process. The major dry processes for NO.sub.x removal are: selective catalytic reduction (SCR) with NH.sub.3, selective (non-catalytic) reduction with NH.sub.3, non-selective catalytic reduction, and adsorption.
Wet processes are usually added downstream of all equipment prior to entering the stack. The major wet: processes for NO.sub.x removal are: absorption with liquid phase oxidation, absorption with liquid phase reduction, and gas phase oxidation followed by absorption.
U.S. Pat. Nos. 3,473,298; 4,681,744; and 4,799,941 disclose processes and devices where exhaust gases are first chilled with direct sprays and thereafter solid contaminants and water soluble substances are removed from the gases by contacting with the water sprays in a spray chamber. The water combines with water soluble gases, such as SO.sub.x, contained in the gases to form sulfurous and sulfuric acids, which are collected with the water spray in a chamber. U.S. Pat. No. 3,881,004 discloses recovery of nitric acid by scrubbing a tail gas with an acid or an alkaline solution which minimizes the discharge of nitrogen oxides to the atmosphere. The publication entitled "Selection of Reactive Solvent for Pollution Abatement of NO.sub.x " by K. R. Jethani et al., Gas Separation & Purification, vol. 4, March 1990, systematically reviews ten chemical reactive systems for removal of NO.sub.x.
A number of U.S. patents disclose NO.sub.x either oxidized or reduced, preceding or following absorption in a liquid solution along with SO.sub.x. Representative U.S. patents disclosing the state of the art include the following. U.S. Pat. No. 4,011,298 discloses removal of NO.sub.x by first oxidizing with ozone and absorbing in acidic solution containing iron compound. U.S. Pat. Nos. 4,541,999 and 4,564,510 disclose oxidation of NO.sub.x to NO.sub.2 by addition of ozone followed by reaction with ammonia forming nitrite and nitrate on absorption and oxidation with air to form nitrate during desulfurization process. U.S. Pat. No. 4,247,321 discloses oxidation of NO.sub.x with ozone and absorbing in lime or limestone or calcium phosphate solution. U.S. Pat. No. 4,107,271 discloses use of iodide solution and ozone. U.S. Pat. No. 3,957,949 discloses use of ozone to convert NO.sub.x to NO.sub.2 and absorbing in reactive medium like sodium chlorite. U.S. Pat. No. 3,997,415 discloses reduction of NO.sub.x and SO.sub.x by irradiation of gas stream. U.S. Pat. No. 4,971,777 discloses oxidation of NO.sub.x containing gas stream with help of organic compounds in the temperature range 300-900.degree. C. then absorbing with ammonia. U.S. Pat. No. 4,119,702 discloses oxidation of NO with ozone, H.sub.2 O.sub.2, chlorine dioxide and nitrogen dioxide followed by reduction with urea. U.S. Pat. No. 4,035,470 discloses a process wherein NO.sub.x containing gases are first oxidized with ozone and chlorine dioxide and then absorbed by sulfite, sulfide, polysulfide or thiosulfate of alkali or alkaline earth metals. U.S. Pat. Nos. 4,999,167; 5,206,002 and 5,316,737 disclose lowering the flue gas temperature near ambient temperature range, i.e 125.degree. F., before mixing with ozone for NO.sub.x abatement. The oxidized contaminants are absorbed in water, or alkaline medium for removal of NO.sub.x.
It is well known that NO is relatively inactive, and at low concentrations, its removal from gas steam is difficult and inefficient. It is also known that NO can be oxidized with many different chemical oxidants to form NO.sub.2. With an oxidant, such as ozone, NO.sub.2 is further oxidized to N.sub.2 O.sub.5, which not only enhances reactivity but also solubility several fold resulting in ease in removal by absorption or adsorption with or without chemical reaction.
A disadvantage experienced with the prior art processes, as disclosed above, is either incomplete oxidation or mixing excessive oxidant in an absorption medium where ozone is wastefully consumed. As disclosed in U.S. Pat. No. 5,206,002, the temperature of flue gas is reduced to near ambient 125.degree. F. before mixing ozone and the required residence time is provided to convert NO.sub.x to N.sub.2 O.sub.5 with stoichiometric quantities of ozone. It is also well known that lowering temperature reduces the rate of oxidation of NO. Further, when ozone and NO.sub.x are both depleted to extinction in the oxidation chamber, the rate of oxidation decreases significantly. In addition, lowering temperature by recovering heat may not be an option when low temperature heat has no gainful use.
Thus, there is need for further improvements in the known oxidative processes to make them commercially viable processes. While oxidation of NO.sub.x to NO.sub.2 is known to improve solubility and reactivity, it is not adequate to remove NO.sub.2 in a cost effective manner. Therefore, there is need for an NO.sub.x removal process by which NO.sub.2 can be further oxidized to a higher state, preferably to NO.sub.3 of at least half of the quantity of NO.sub.x.
Further, there is need to provide a process and an apparatus for removing contaminants, specifically NO.sub.x and SO.sub.x emissions, from exhaust gas that can be applied to any combustion (low or high sulfur coal fired, gas fired or oil fired, incineration, furnaces) or flue gas from chemical process system. NO.sub.x and SO.sub.x must be removed simultaneously but independently of one another. The process must not be dependent upon capital intensive equipment. The process must not cause scaling or lose performance with time. The abatement process must not produce secondary emissions or hazardous products and must be operable in a wide temperature range. The process must also be applicable for varying compositions of NO.sub.x while at the same time reduce the content of the contaminants in the exhaust gases to the required levels as prescribed by air quality regulations. An improved emission system is therefore required that not only brings the content of the contaminants in the exhaust gases into compliance with regulated air quality standards, but also has the capability to meet future standards.